Advertisers and merchants desire the ability to display graphic and/or informational images on a wide variety of surfaces. In recent years, transparent surfaces such as, windows, walls, and the like, have attracted a great deal of attention as substrates for advertising media. If a graphic image is applied to a transparent substrate, it is desirable that the image be visible when viewed from one side of the window while leaving the window and image substantially transparent when viewed from the other side of the window. Thus, for example, if the image is to be mounted on a window of a vehicle, such as a bus or taxicab, it is desired that passengers be able to see clearly through the window, while pedestrians outside of the vehicle see the graphic image.
Graphic articles achieving the effect typically are multi-layer film constructions with an opaque (light colored, usually white) film adapted to receive an image on one surface and a light absorbing (dark, usually black) film or ink applied to the opposite surface. A pressure sensitive adhesive and removable backing, or carrier, material is applied to the dark film or ink which allows the printed graphic to be handled and applied to a window.
Numerous perforations through the film layers create an optical illusion of "transparency" through the graphic article. The perforations are sized and spaced such that, when observed from the imaged side, a viewer has a tendency to focus on the image. However, when observed from the relatively darker side, the viewer has a tendency to see through the graphic article, leaving the window unobstructed. In addition, it is well known that windows appear dark or opaque when viewed from an area of relatively bright ambient light into a relatively darker area. When viewed from an area of relatively dark ambient light into a relatively brighter area, the windows appear transparent. The unidirectional effect of the graphic article is enhanced by this effect, which allows viewers in relatively darker areas, such as the interiors of vehicles and buildings, to see through the unidirectional graphic article, while viewers in relatively brighter ambient light will see the printed graphic.
Parties other than the film manufacturer typically image unidirectional graphic articles of the type described above. Since the film cannot be imaged through the adhesive and backing material, the image and backing material must be applied to the light side of the film. The graphic articles are then mounted on a window so that the imaged surface may be viewed directly and the dark surface may be viewed through the glass (referred to as a "first surface" application). As a result, when the window is a barrier between the indoors and the outdoors, the unidirectional film must be applied to the exterior of the window with the imaged surface exposed to the outside environment. Unfortunately, in some applications, exposure of the imaged surface of the graphic article to the environment is not ideal since the image must be durable, resistant to ultraviolet and other ambient light, and weatherable. Additionally, dirt and other contaminants may become entrapped within the perforations of the film and adhere to the pressure sensitive adhesive against the window, reducing the service life of the graphic article. Although application of a clear, un-perforated protective layer over the imaged surface may improve service life under certain conditions, such layers are expensive and require additional process steps that may be difficult for the end-user to perform.
To avoid the problems created by exposure of the imaged surface of the graphic article to the environment, it is desirable that the image be viewable through the transparent substrate (referred to as a "second surface" application). For example, the graphic article may be applied to the interior of a window so that the image may be viewed through the window glass by observers outside a building. However, to occupants of the building, the view outward through the window glass will be relatively unobstructed. This mounting procedure allows the window glass itself to act as a protective barrier for the imaged surface of the graphic article.
WO 96/11798 illustrates in FIG. 6C an example of an interior mount graphic article with an image that may be viewed unidirectionally. This article includes a single transparent panel 12 with a color image layer 22 applied on a first surface. The second surface of the image layer 22 includes an adhesive layer 48 that may be protected by an optional peel-off liner layer 50. A light absorbing (black) paint layer 24 is applied over the image layer 22. The entire construction is perforated with through holes 42. As shown in FIG. 6D of WO '798, the liner layer 50 may be removed and the perforated construction may be attached to an interior surface of a window using the adhesive layer 48.
Unidirectional graphic articles sold by Cadillac Plastic and Chemical Co. under the trade designation Interior Mount Clear Focus Film include a transparent, perforated poly (vinyl chloride) (PVC) reverse imaged on one surface. The surface of the PVC film opposite the image includes a pressure sensitive adhesive layer that may be used to mount the graphic article on an interior surface of a window. After imaging, the imaged surface of the PVC layer is screen printed (flood coated) first with an opaque layer of white ink, and next is screen printed (flood coated) with a layer of black ink. Following the flood coating steps, the graphic article may be applied to an interior surface of a window with the pressure sensitive adhesive.
U.S. Pat. No. 5,769,436 to Andriash illustrates in FIG. 2 a unidirectional interior mount graphic article 14 with a retro-reflective opaque sheet 12. The opaque sheet 12 is imaged on a first surface and includes a dark adhesive layer 15 on a second surface. A double-sided transparent transfer adhesive 13 is placed over the image surface of the sheet 12 after printing. The composite structure is then perforated with a laser. A clear laminate 19 may be adhered to the black adhesive layer 15 to seal the structure and prevent entry of dirt into the perforations. The graphic article 14 may then be mounted on an interior surface of a window with the transfer adhesive 13.
It is well known in the signage industry that advertisers and merchants prefer graphic articles that may be prepared at a local print shop. This ensures that the articles will be easily updateable and printable with a wider variety of custom images at a reasonable cost. While the currently available interior mount unidirectional graphic articles perform well, these articles are difficult for an end user to prepare with commonly available printing equipment. Applying a dark paint layer on the image as in WO '798, or flood coating white and/or black layers over the image, requires specialized printing equipment and inks that are unavailable and/or too expensive for many end users. In addition, unless specialized inks and screen printing equipment are used, in many cases these coating steps do not adequately cover the image layer, which results in a less than satisfactory image. The construction described in the '436 patent avoids these problems, but requires laser perforation following the imaging step. This is not feasible for the typical end user.